1. Field of the Invention
The present invention relates to an internal release agent, a method for molding a resin using the internal release agent and a molded article obtained therefrom.
2. Description of the Related Art
There are known various methods for molding resins such as casting molding, injection molding and the like. In each method, so-called release agents are used for improving releasability between a molded article and a mold.
In particular, urethane resin, epoxy resins and the like have a strong adhesion and therefore, it is essential to use a release agent.
Release agents are generally classified into two groups, that is, external release agents and internal release agents.
External release agents should be applied to the inner surface of a mold with each molding procedure. Therefore, there are various problems as shown below. The productivity of the molded articles is poor, an external release agent is transferred onto the surface of the resulting molded articles resulting in irregular portions on the surface of the molded article and causing defective coating and dyeing. Furthermore, a transparent resin molded article becomes turbid, and the like.
On the contrary, an internal release agent is added to a resin producing monomers in advance. It is not necessary to apply it to a mold with each molding. Therefore, an internal release agent favors improvement in productivity.
As internal release agents, there are known internal release agents for urethane resin molding, for example, fluorine type nonionic surface active agents, silicone type nonionic surface active agents, alkyl quaternary ammonium salts, acid phosphoric esters, liquid paraffin, waxes, higher fatty acids and metal salts thereof, higher fatty acid esters, higher aliphatic alcohols, bisamides, polysiloxanes, aliphatic amine ethylene oxide adducts and the like (Japanese Patent Application Laid-open No. Hei 1-295201).
Phosphoric acid esters having polyether substituents have been recently proposed (Japanese Patent Application Laid-open No. Hei 3-287641).
For reaction injection molding of urethane resins, zinc stearate is generally used, but zinc stearate is not so soluble in the starting materials that various compatibilizers have been proposed (Japanese Patent Application Laid-open No. Hei 3-273030 and Japanese Patent Publication No. Hei 3-27586).
In the molding of epoxy resins, there are known a method using zinc stearate as an internal release agent (Japanese Patent Application Laid-open No. Hei 1-213602), a method using fluorine type nonionic surface active agents, silicon type nonionic surface active agent, alkyl quaternary ammonium salts, acid phosphoric esters and the like as internal release agents (Japanese Patent Application Laid-open No. Hei 3-81320), and methods of using both an external release agent and an internal release agent (Japanese Patent Application Laid-open Nos. Sho 63-144302 and 63 - 144303).
Further, in the molding of polyolefin resins, there are used aliphatic alcohols, aliphatic esters, phthalic acid esters, triglycerides, fluorine type surface active agents, higher fatty acid metal salts and the like as internal release agents (Japanese Patent Application Laid-open Nos. Hei 2-84409 and 2-44301).
In the molding of polycarbonate resins, there are used higher fatty acid esters, waxes, liquid paraffin, silicone oil, alkyl glyceryl ethers and the like as internal release agents (Japanese Patent Application Laid-open Nos. Hei 1-315460 and 1-315459).
For molding acrylic resins, there are generally used molding methods where higher alcohols, higher fatty acid esters, higher fatty acids and the like are used as internal release agents (Japanese Patent Application Laid-open No. Hei 2-105844).
The known internal release agents as above are effective and improve productivity of the molded articles as compared with external release agents. However, the releasing function is not sufficient and thereby, the resins sometimes remain on the surface of molds and in the case of molds made of glass, glass sometimes peeled off. In addition, excess stress is applied due to the resistance upon releasing and thereby the molded article is liable to warp or to be strained.
When internal release agents are used in an amount sufficient to release satisfactorily, the molded articles are liable to become turbid.
Such drawbacks are fatal to molding optical products such as plastic lenses particularly requiring surface accuracy and transparency, light discs and the like, and deteriorate the commodity value to a great extent.
In view of the foregoing, development of a new internal release agent has been strongly demanded which has a sufficient releasing function and does not deteriorate transparency in the case of molding a transparent resin.